Traditional electrically-small antennas (ESA) may enable a broad-band operation from non-resonant loop antennas. Some efforts may employ voltage-mode Class-D power amplifiers (PA) using Radio Frequency Pulse Width Modulation (RF-PWM) to synthesize low-harmonic loop current.
While an improvement over traditional use of Class-AB PA to perform this task, the efficiency of these schemes may be quite low, not exceeding the ratio of the radiation resistance of the loop to twice the resistance (Rds) on of the switching devices. The switching losses of this scheme are expected to further degrade system efficiency.
One such traditional attempt may be described in U.S. Pat. No. 6,614,403B1 to Merenda. Because of the high voltage necessary to generate antenna current against the loop inductance, Merenda proposes placing numerous switch modules in series to sum the breakdown voltage of individual switching devices.
The concept of the Digital Wideband Small Antenna System, devised by Merenda, is to utilize a non-resonant, electrically small loop antenna as a wideband antenna. By driving it with a pulse-width-modulated switched-voltage signal, one may attempt to synthesize current waveforms within the loop to permit wideband operation.
However, the concept requires low-loss, fast switching RF power devices, to take full advantage of the approach. The losses resulting from finite switching device ON resistance attempting to supply power into vanishingly small loop radiation resistances, combined with the Digital Signal Processor (DSP) power needed to generate the RF-PWM signals severely limit the efficiency of the concept.
Additional attempts to overcome these shortfalls may offer a method of operating an electrically small antenna in a broadband manner by driving a magnetic dipole loop structure with a stepped voltage to synthesize a current waveform. While it is possible to operate an electrically small magnetic dipole antenna over multiple octaves of bandwidth, the radiated pattern is irregular, comprised of both horizontal and vertical components due to the voltage driving the feed point of the loop. In addition, the power required to overcome the inductive reactance of the loop feed point and sufficiently resonate the loop may require multiple power amplifiers to create sufficient power into the loop antenna to force radiation.
A switching power amplifier enabling waveform synthesis may be best described in U.S. Pat. No. 4,894,621 which is incorporated by reference in its entirety. This PA enables a third and fifth harmonic cancellation through voltage level manipulation in a switching power amplifier. This technique, while applicable for cancellation of signal harmonics, requires a complicated power amplifier configuration.
However, a loop antenna, as described by the previous references, when driven by a voltage source at a feed point along its circumference will possess a non-uniform directional radiation pattern over frequency, affecting the ability to cancel harmonic radiation from the harmonically-rich switched voltage source.
These attempts fail to address that effectiveness of the loop antenna increases with frequency up to the resonant frequency of the loop or a harmonic of the resonant frequency of the loop. The square wave output of a class-D PA may contain all odd harmonics in relatively high amplitude. To simply apply a class-D PA to an ESA one may struggle to force radiation at the desired frequency below resonance while those harmonics which fall near the frequencies of resonance of the loop will be radiated at a significantly higher amplitude. Therefore, such a system may not meet a nominal harmonic radiation specification (e.g. −80 dBc).
Consequently, a need remains for a wideband voltage driven electrically small antenna able to overcome the harmonic limitations and additional shortfalls of the prior art systems.